The present invention relates to a fuel evaporator to be installed between a carburetter and an intake manifold of an internal combustion engine for heating an air-fuel mixture flowing through an air-fuel passage of the carburetter. The above described fuel evaporator is provided with a honeycomb-shaped ceramic heater made of semi-conductive material having a positive temperature coefficient of resistance. The ceramic heater is disposed on a stepped portion formed in an insulator which is interposed between the carburetter and the intake manifold. Conventionally, in such a fuel evaporator, a square PTC ceramic plate has been generally used, since the square ceramic plate can be closely fit in the square stepped portion of the insulator without slipping on the insulator during operation.
However, in this case, four corners of the square stepped portion project from the air-fuel passage outward to approach four holes through which bolts are inserted for fixing the fuel evaporator to the carburetter and the intake manifold.
As a result, a large sealing surface cannot be obtained between the bolt holes and the stepped portion so that sealing effect of the fuel evaporator remarkably lowers.
By employing a ceramic heater having a circular outer shape, the above described problem can be overcome. However, since the circular heater is disposed in a stepped portion having a circular cross section, which is formed in the insulator, the circular ceramic heater is apt to slip relative to the insulator due to vibrations of an engine so that the circular ceramic heater is damaged.
Therefore, the circular ceramic heater is inferior in durability.
Accordingly, one object of the present invention is to provide a fuel evaporator for an internal combustion engine having excellent durability, which can be installed in a limited space with excellent sealing effect.
Another object of the present invention is to provide a fuel evaporator for an internal combustion engine, having a ceramic heater which is surely fixed to an insulator during operation.
Still another object of the present invention is to provide a fuel evaporator for an internal combustion engine, having a ceramic heater which is free from damage such as cracks.